In general, molten iron is subjected to preliminary treatment for preliminarily removing silicon and phosphorus in the molten iron prior to decarburization refining in a converter. The preliminary treatment of the molten iron is carried out for the purpose of decreasing an amount of a refining flux used, enhancing a purity of molten steel, improving a yield of manganese by prevention of peroxidation in an operation with a converter, decreasing an amount of refining slag, and so on, and various methods including a treatment in decarburization step are proposed therefor.
The refining slag is generated in the preliminary treatment of the molten iron. When the refining slag is utilized for various applications, it is required that fluorine and the like are not eluted depending on the application. To this end, there has hitherto been reviewed a method of conducting preliminary treatment of molten iron using no fluorite (CaF2) as a fluorine source used for the purpose of enhancing dephosphorization efficiency. Recently, it is also demanded to reduce volume of greenhouse gas emission in steel industry. In this connection, there is reviewed a refining method wherein usage rate of molten iron from a blast furnace requiring large energy for reduction of iron oxide is decreased while usage rate of cold iron source such as iron scrap or the like is increased. Under such a situation, the recent method for the preliminary treatment of molten iron tends to increase the usage rate of the cold iron source while improving the refining method.
As one of preliminary treatments of molten iron conducting desiliconization or dephosphorization of molten iron, there is a method wherein silicon or phosphorus in the molten iron is removed into a slag by adding a refining agent (flux solvent) such as quicklime or the like to the molten iron while adding a gaseous oxygen or a solid oxygen source such as iron oxide or the like. As a container for the treatment of molten iron are used a transportation container such as torpedo car, blast furnace pan or the like, a converter type container (refining furnace) and so on. In order to use a great amount of scrap, it is advantageous to use a converter type container having a large capacity.
In Patent Document 1 is disclosed a refining method of molten iron conducting desiliconization and dephosphorization with a converter wherein intermediate slag removal is carried out after the desiliconization in the converter and subsequently dephosphorization is conducted. This method proposes a method wherein rephosphorization after the desiliconization is suppressed by controlling the composition of the slag to facilitate subsequent dephosphorization.
In Patent Document 2 is disclosed a method for preliminary treatment of molten iron wherein desiliconization is carried out with a converter type reaction vessel and the desiliconized molten iron after the tapping and slag removal is again returned to the reaction vessel to conduct dephosphorization. This method for preliminary treatment of molten iron is a method of conducting dephosphorization efficiently by properly adjusting silicon concentration, slag basicity and iron oxide concentration in the molten iron after the desiliconization without using fluorite and a method of dissolving scrap in the desiliconization and/or dephosphorization.
In Patent Document 3 is disclosed a preliminary treatment method continuously conducting desiliconization and dephosphorization of molten iron with a converter type container wherein 40˜60 mass % of a slag generated after the dephosphorization of a precedent charge is retained in the container and utilized in the desiliconization and dephosphorization to decrease the amount of slag generated.